Apple Watch workout app

If the Fitness App on your iPhone has not been moved to the App Library on the iPhone, you can open the AppStore on your iPhone, type "Fitness app Apple" init the search field, then choose the Apple Fitness app to be installed again:

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Restore a built-in app that you deleted

1. On your iOS or iPadOS device, go to the App Store.
2. Search for the app. Make sure that you use the exact name of the app. Find the correct name of built-in apps.
3. Tap  to restore the app.
4. Wait for the app to restore, then open it from your Home screen.

If you have an Apple Watch, restoring an app to your iPhone also restores that app to your Apple Watch.

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copied from: Delete built-in Apple apps on your iOS 12, iOS 13, or iPadOS device or Apple Watch - Apple Support

Is this about a 3rd party workout app?

If you already bought the app in the past, open the AppStore on your Apple Watch, search for the app in question and download it again.

Check the steps in this article:

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1. Open the App Store app  on your Apple Watch.
2. Turn the Digital Crown to browse featured apps.
3. Tap a category or tap See All below a collection to see more apps.
4. To get a free app, tap Get. To buy an app, tap the price.
5. If you see  instead of a price, you’ve already purchased the app and you can download it again without a charge. Some apps require that you also have the iOS version of the app on your iPhone.

To find a specific app, tap the Search field at the top of the screen, then type (Apple Watch Series 7 and Apple Watch Series 8), or use dictation or Scribble to enter the name of the app. You can also browse trending categories of apps by tapping a category.

To use Scribble on Apple Watch Series 7 and Apple Watch Series 8, swipe up from the bottom of the screen, then tap Scribble.

Note: When using Apple Watch with cellular, cellular data charges may apply. Scribble is not available in all languages.

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Install apps you already have on iPhone

By default, apps on your iPhone that have a watchOS app available are automatically installed and appear on the Home Screen. To instead choose to install specific apps, follow these steps:

1. Open the Apple Watch app on your iPhone.
2. Tap My Watch, tap General, then turn off Automatic App Install.
3. Tap My Watch, then scroll down to Available Apps.
4. Tap Install next to the apps you want to install.

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copied from the Apple Watch User Guide

You're welcome 🙂